Heterogeneous immunoassays typically require the separation of sought-for components bound to component-selective particles from unbound or free components of the assay. To increase the efficiency of this separation, many assays wash the solid phase (the bound component) of the assay after the initial separation (the removal or aspiration of the liquid phase). Some chemiluminescent immunoassays use magnetic separation to remove the unbound assay components from the reaction vessel prior to addition of a reagent used in producing chemiluminescence or the detectable signal indicative of the amount of bound component present. This is accomplished by using magnetizable particles including, but not restricted to, paramagnetic particles, superparamagnetic particles, ferromagnetic particles and ferrimagnetic particles. Tested-for assay components are bound to component-specific sites on magnetizable particles during the course of the assay. The associated magnetizable particles are attracted to magnets for retention in the reaction vessel while the liquid phase, containing unbound components, is aspirated from the reaction vessel.
Washing of the solid phase after the initial separation is accomplished by dispensing and then aspirating a wash buffer while the magnetizable particles are attracted to the magnet.
Greater efficiency in washing is accomplished by moving the reaction vessels along a magnet array having a gap in the array structure proximate a wash position, allowing the magnetizable particles to resuspend during the dispense of the wash buffer. This is known as resuspension wash. Subsequent positions in the array include magnets, allowing the magnetizable particles to recollect prior to aspiration of the wash buffer and introduction of reagent beyond the end of the magnet array.
One prior art wash block configuration provides an aluminum insert in the gap of the magnet array at the wash position. Rather than simply removing a magnet from the resuspension position, the insert prevents a reaction vessel from becoming misaligned and jammed in the magnet array. While functioning adequately for assays which employ resuspension wash, it is evident that the provision of an aluminum insert in place of a magnet at the wash position adversely effects assays which do not use the resuspension in washing but which proceed through the wash position without resuspension. A single band of magnetizable particles which is normally formed along the interior of the reaction vessel as it passes the magnet array, during the initial separation, is split into two smaller bands on either side of the reaction vessel due to attraction by the magnets on either side of the insert at the resuspension and wash position. Since the reagent is introduced into the reaction vessel in a stream directed toward where the magnetizable particles collected before splitting, the split in the banding of the magnetizable particles results in the stream missing the main concentration of magnetizable particles. Poor resuspension of the magnetizable particles during resuspension wash and upon addition of an acid reagent used to condition the bound component reagent in the generation of a chemiluminescent signal results.
Therefore, the prior art fails to provide a wash region which enables the efficient washing of magnetizable particles during the wash phase of a magnetic separation assay without adversely effecting assays not employing resuspension wash.